1. Introduction
Our current API runs on Nancy which in my opinion is past its prime. Recent news from the GitHub issue tracker seems to confirm that thesis, that is why we started looking for a migration path from Nancy based API to ASP.NET Core. Because the codebase is quite large we didn’t want to do a Big Bang Rewrite but instead of that, we wanted to gradually replace old API with a new one, so both of them can coexist next to each other.
2. Legacy API
Before I jump into implementation, here is a sample Nancy API with two endpoints – products and variants
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public class Startup { // This method gets called by the runtime. Use this method to add services to the container. public void ConfigureServices(IServiceCollection services) { // nancy has its own container } // This method gets called by the runtime. Use this method to configure the HTTP request pipeline. public void Configure(IApplicationBuilder app, IWebHostEnvironment env) { app.UseOwin(owin => owin.UseNancy()); } } |
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public sealed class ProductsNancyModule : NancyModule { public ProductsNancyModule() { Get("products", args => Response.AsJson(new List<Product> { new Product(Guid.NewGuid().ToString(), "First product from nancy"), new Product(Guid.NewGuid().ToString(), "Second product from nancy") })); } } |
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public sealed class VariantsNancyModule : NancyModule { public VariantsNancyModule() { Get("variants", args => Response.AsJson(new List<Variant> { new Variant(Guid.NewGuid().ToString(), Guid.NewGuid().ToString(), "first variant from nancy"), new Variant(Guid.NewGuid().ToString(), Guid.NewGuid().ToString(), "second variant from nancy") })); } } |
The goal is to replace products endpoint with ASP.NET Core implementation while variants endpoint should still be served by Nancy
3. Combining Nancy with ASP.NET Core
The solution is based on branching the pipeline feature which is available starting from ASP.NET Core 2.1. Long story short – it is possible to configure different pipelines for different route paths thanks to
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public static IApplicationBuilder Map(this IApplicationBuilder app, PathString pathMatch, Action<IApplicationBuilder> configuration) |
method. Having that in mind we can Map products path to run through ASP.NET Core pipeline whereas the rest would go through Nancy.
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// This method gets called by the runtime. Use this method to add services to the container. public void ConfigureServices(IServiceCollection services) { // register mvc services services.AddControllers(); services.AddMvc(options => options.EnableEndpointRouting = false); } // This method gets called by the runtime. Use this method to configure the HTTP request pipeline. public void Configure(IApplicationBuilder app, IWebHostEnvironment env) { // run everything with products prefix through mvc app.Map("/products", appBuilder => { appBuilder.UseMvc(); }); // run the rest through Nancy app.Map(string.Empty, appBuilder => { appBuilder.UseOwin(options => options.UseNancy()); }); } |
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[ApiController] public class ProductsController : ControllerBase { [HttpGet("/products")] public List<Product> Get() { return new List<Product> { new Product(Guid.NewGuid().ToString(), "First product from asp net core"), new Product(Guid.NewGuid().ToString(), "Second product from asp net core") }; } } |
At this point we are almost there, however running request through Map pipeline will remove the path prefix, meaning that our controller would be accessible under / path instead of products. In order to bypass this limitation we have to restore original prefix with RewriteMiddleware. Once we put it all together, now we are able to replace multiple Nancy endpoints with ASP.NET Core ones using following piece of code
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// This method gets called by the runtime. Use this method to configure the HTTP request pipeline. public void Configure(IApplicationBuilder app, IWebHostEnvironment env) { var aspNetCorePaths = new PathString[] {"/products"}; foreach (var path in aspNetCorePaths) { app.Map(path, appBuilder => { // we still want to access the resource under old url, so we need to add the branch path back to the route var rewriteOptions = new RewriteOptions().AddRewrite( "(?<Path>.*)", $"{path}/${{Path}}", skipRemainingRules: true); appBuilder.UseRewriter(rewriteOptions); appBuilder.UseMvc(); }); } app.Map(string.Empty, appBuilder => { appBuilder.UseOwin(options => options.UseNancy()); }); } |
Source code for this post can be found here